This invention relates to a method of preventing the generation of a ground circulating current in an induction heating apparatus for molten steel in a tundish in a continuous casting operation.
Superheating temperatures (hereinafter referred to as "SH") of molten steel in a tundish in a continuous casting operation (see FIG. 6) greatly influence the quality of a steel product. If SH is high, this interrupts equiaxis crystallization, and increases central segregation. If SH is low, the viscosity of the molten steel increases, and defects due to inclusions frequently occur, for example, as a result of generation of deckel (i.e., agglomerates formed by inclusions or powder deposited on the molten steel) in a mold. Therefore, it is desirable from the viewpoint of the quality that SH should always be controlled to a target range. Generally, however, when trying to keep SH constant in a charge under the influence of heat dissipation from the molten steel in a ladle, the casting speed must be varied. This has resulted in a problem that the efficiency of the production is lowered, and besides the lowering of SH at a final stage of the casting operation could not be sufficiently compensated for.
Under the circumstances, there has now been made an attempt in which a heating function is added to a tundish for continuous casting to prevent the lowering of SH at an initial stage and a final stage of the casting operation. For heating molten steel in a tundish, it is a common practice to use an electrically-operated induction heating system in view of HS controllability and economy.
A commonly-used induction heating apparatus will now be described with reference to FIG. 4. Reference numeral 1 denotes a two-leg iron core. A tundish 20 is enclosed by an iron shell 13 and is lined with a refractory. This tundish is divided into first and second chambers 6 and 7 which communicate with each other by molten steel channels 14 and 15. The first and second chambers 6 and 7 are separated from each other by a refractory wall 4, and one leg of the two-leg iron core 1 extends through a central portion of the refractory wall 4, and a coil 2 is wound on the one leg of the two-leg iron core 1. A loop current circuit is formed through molten steel 5 around the two-leg iron core 1 (which constitutes a closed magnetic field circuit) through the molten steel channels 14 and 15 (which are provided to interlink the two-leg iron core 1) within the tundish 20. A ground circuit 18 serves to effect grounding via the molten steel, the refractory and the associated equipment from the first chamber 6, and a ground circuit 19 serves to effect grounding via the molten steel, the refractory and the associated equipment from the second chamber 7. Molten steel is supplied from a ladle 9 (FIG. 6) into the first chamber 6, and the molten steel is fed from the second chamber 7 into a continuous casting mold 11 through a submerged entry nozzle.
The condition of the molten steel in the tundish and the ladle, the condition of the molten steel discharged to the mold, and electric circuits will now be described with reference to FIG. 5 which is a perspective view.
In this case, the molten steel receiving chamber (first chamber) 6 of the tundish 20 is electrically connected to the ladle 9 via the molten steel or a long nozzle 8, and the ladle 9 is connected to the earth 18 via a ladle support. The molten steel discharging chamber (second chamber) 7 is connected to the mold 11 or a pinch roll 12 via the molten steel or an immersion nozzle 10, and the mold 11 or the pinch roll 12 is connected to the ground 19.
In FIG. 5, it can be considered that the electric circuits are a loop 25 via the molten steel, a loop 17 via the iron shell of the tundish (the loop 17 being formed in iron shell 13), and a ground circuit 16. In FIG. 5, reference numeral 13 denotes the iron shell of the tundish, and reference numeral 1 denotes the two-leg iron core.
Rz: Iron shell insulating plate (Rz&gt;0.OMEGA.) PA1 Rx: LD (ladle) earth resistance (Rx=several tens of m.OMEGA.) PA1 R.sub.1 : LN (long nozzle) refractory resistance (R.sub.1 &gt;0.OMEGA.) PA1 Rs: PR (pinch roll) resistance (Rs=several tens of m.OMEGA.)
Here, the ground circuit 16 is to be considered.
An earth circulating current flows through this ground circuit 16, and problems, such as red heat and electrolytic corrosion of the equipment, have often been encountered.
In FIG. 4, when the first chamber 6 and the second chamber 7 are connected to the ground, an electric circuit serving as a ground circuit (indicated by a broken line) is formed in addition to a molten steel-heating circuit (indicated by a solid line) which is an originally-intended function, and a ground circulating current flows therein. To avoid this circulating current, there has been used a method of insulating the ground circuit; however, this has required much time and labor for the maintenance of the facilities.